Pattern regulation in the ventral histoblasts of the housefly: induction of sternal pattern abnormalities by mechanical wounding of larval epidermal cells

Comparative analysis reveals the complex role of histoblast nest size in the evolution of novel insect abdominal appendages in Sepsidae (Diptera)

BACKGROUND

The males of some sepsid species (Sepsidae: Diptera) have abdominal appendages that are remarkable in several ways. They are sexually dimorphic, have a complex evolutionary history of gain and loss, and can be jointed and thus highly mobile. The sternite brushes are used extensively in complex courtship behaviors that differ considerably between species and during mating. The abdominal appendages have a novel developmental pathway developing from histoblast nests rather than imaginal discs.

RESULTS

We focus on the evolution of cell number, nest area, and segment length in both sexes to understand how this tissue relates to the formation of novel abdominal appendages. We map histoblast nest size of wandering-phase larvae of 17 species across 10 genera to a phylogenetic tree of Sepsidae and demonstrate that abdominal appendages require significant increases of histoblast nest size and cell number in most species while one species produces small appendages even without such modifications. In species with particularly large appendages, not only the nests on the fourth, but nests in neighboring segments are enlarged (Themira biloba, Themira putris). The loss of abdominal appendages corresponds to the loss of an enlarged fourth histoblast nest, although one species showed an exception to this pattern. One species that constitutes an independent origin of abdominal appendages (Perochaeta dikowi) uses an unusual developmental mechanism in that the histoblast nest sizes are not sexually dimorphic.

CONCLUSIONS

The surprisingly high diversity in histoblast size and degree of sexual dimorphism suggests that the developmental mechanism used for abdominal appendage formation in sepsids is highly adaptable. The presence of appendages usually correlate with increased histoblast cell number and in most cases appendage loss results in a return to ancestral histoblast morphology. However, we also identify several exceptions that indicate the abdominal appendages have a malleable developmental origin that is responsive to selection.

Deciphering the evolutionary history and developmental mechanisms of a complex sexual ornament: the abdominal appendages of Sepsidae (Diptera)

Male abdomen appendages are a novel trait found within Sepsidae (Diptera). Here we demonstrate that they are likely to have evolved once, were lost three times, and then secondarily gained in one lineage. The developmental basis of these appendages was investigated by counting the number of histoblast cells in each abdominal segment in four species: two that represented the initial instance of appendage evolution, one that has secondarily gained appendages, and one species that did not have appendages. Males of all species with appendages have elevated cell counts for the fourth segment, which gives rise to the appendages. In Perochaeta dikowi, which reacquired the trait, the females also have elevated cell count on the fourth segment despite the fact that females do not develop appendages. The species without appendages has similar cell counts in all segments regardless of sex. These results suggest that the basis for appendage development is shared in males across all species, but the sexual dimorphism is regulated differently in P. dikowi.

Negative growth control of mitotically active imaginal cells (histoblasts) of the abdominal epidermis during metamorphosis of the housefly

On the ventral side of each pupal abdominal segment of the housefly, there is a pair of histoblast nests, each containing about 600 diploid cells. These cells, during adult development, divide, replace intervening polytene larval epidermal cells (LEC), and form both the median sternite and the surrounding pleura of the adult segment. Since the histoblast nests and the LEC form a contiguous layer, we examined the role of these two types of cells in regulating the mitotic potential of the histoblasts during development of the median sternite. Two experimental approaches were used: deletion of one of the nests by thermocautery; and by disturbance of the continuity of the monolayered epidermis by thermocautery of, or topical application of heptanol on, the midventral LEC. Ablation of one of the contralateral nests resulted in a mirror image duplication of the hemisternite and pleura by the surviving nest. Disturbance of the continuity of the LEC produced mirror image duplication of the hemisternal pattern by each of the contralateral nests. From these results, we propose that the contralateral ventral nests mutually downregulate their mitotic potential by secreting regulatory factor(s) to produce the normal median sternite pattern and surrounding pleura. We also suggest that these chemicals act in a paracrine fashion, possibly through gap junctions in the LEC. © 1994 Wiley-Liss, Inc.

What are and what are not imaginal discs: reevaluation of some basic concepts (Insecta, Holometabola)

Some general aspects of the concept of imaginal discs in the Holometabola are reevaluated. Their monolayer character and continuity with the surrounding epidermis are confirmed. Studies on the imaginal discs of the silkworm (Bombyx mori) and data from the literature show that the discs and their peripodial cells produce cuticle during larval life, as well as at metamorphosis. In B. mori it is demonstrated that adult and larval antennae are produced by the same cells or their progeny. The results also suggest that segments of the typically three-segmented larval antenna of Holometabola are not scape, pedicel, and one-segmented flagellum; at least segments 2 and 3 are of flagellar origin. Based on these and some additional facts it is argued that: (1) No larval organs are "replaced" at metamorphosis, but strict "sequential homology" is always maintained. (2) Imaginal discs are not undifferentiated structures destined to form the adult after larval breakdown, cannot be unambiguously defined, and do not represent qualitatively different epidermal structures. Classical imaginal discs (invaginated and present also in pre-final larval instars) arose several times independently and were not present in the larvae of ancestral Holometabola. (3) Since the disc cells are not undifferentiated and "embryonic" (if these words have a defined meaning at all), it is unreasonable to expect that the processes taking place in discs at metamorphosis would differ fundamentally from those occurring in other diploid metamorphosing epidermal cells.

The Ultrabithorax gene of Drosophila and the specification of abdominal histoblasts

Separation of the imaginal and larval developmental pathways in Drosophila occurs early in embryogenesis, resulting in the formation of imaginal discs and abdominal histoblast nests along the larval body wall. The dorsal and ventral histoblast nests within the first abdominal (A1) segment are shown not to be segmentally homologous with the metathoracic (T3) haltere and leg discs, respectively, since they occur at distinct dorso-ventral locations during normal development and can be found together within the same segment in mutants of the Bithorax complex (BX-C) where T3 is transformed towards A2-A4 or A1 towards T3. Several patterning abnormalities are also observed in BX-C mutants. A ventral shift in the A1 ventral nest occurs in partially transformed larvae harboring weak bithoraxoid (bxd) mutations; in more fully transformed larvae (Ubx1/Df) both the anterior dorsal and ventral nests are lost and instead a dorsal and ventral disc bud are formed. Dorso-ventral inversions in the pattern of the ventral nest occur in a random fashion throughout A1-A7 in response to an increase or decrease in the gene dosage of the BX-C. In gain-of-function mutants anterior dorsal histoblast cells form in the homologous anterior as well as the nonhomologous posterior portion of T3. Based on these and other findings it appears that the Ultrabithorax (Ubx) locus (and possibly abdominal-A and Abdominal-B) is required to steer ectodermal cells toward an imaginal histoblast rather than a larval cell fate at specific regions within the first abdominal segment.